UT Experiment May Alter Understanding of Crystals

KNOXVILLE — Experiments by University of Tennessee scientists may bring a new level of understanding to theories about how solids change state as they cool.

In the Sept. 24 Science magazine, a team of UT-Knoxville researchers led by Dr. Hanno Weitering describe how a two-dimensional layer of tin changes as its temperature is lowered.

The finding should allow theoretical physicists to create better mathematical models of how an element moves from one solid state or phase to another, said Weitering, assistant professor of physics.

Physicists know that impurities or defects somehow play an important role in phase changes, but current theories assume that impurities in a crystal are positioned randomly and are immobile at low temperatures, Weitering said.

The UT experiment suggests otherwise, he said.

When a layer of tin one atom deep is cooled, the change of state starts around any impurities in the tin crystal and spreads out like a wave in a pond toward other impurities, Weitering said.

In this experiment, the impurities are atoms of germanium that are present in the crystal structure of the tin. When the waves collide, atoms of tin and germanium suddenly switch place, repositioning the impurities in a crystal lattice.

“For the first time, we’ve shown how an impurity initiates a phase transition and that the random impurities start to become ordered at low temperature,” Weitering said.

Graduate student Anatoli Melechko used a scanning tunneling microscope to look at individual atoms as the tin crystal was cooled.

States or phases are words physicists use to describe matter in its different manifestations, for example, solid, liquid and gas. Some elements like tin have more than one crystalline phase, depending on temperature and other variables.

In addition to Weitering and Melechko, the team included Dr. E. Ward Plummer, UT-Oak Ridge National Laboratory Distinguished Scientist in physics and Miroslaw Bartkowiak, UT research associate. The National Science Foundation sponsored the research.